With the development of the mobile market, there is an increasing demand for higher audio quality. One important factor for high audio quality is volume, which is directly related to the excursion of the transducer speaker. If the speaker is moving so much that it collides with the outer enclosure of the mobile device, unpleasant or distorted audio effects might occur.
With an increase in the number of mechanical parts and semiconductor parts integrated inside a device such as a mobile device, while still maintaining a relatively smaller physical design of the whole device, it may be important for the size of the individual parts within the device to be minimized.
FIG. 1a illustrates an example of a mobile device 100 according to the prior art.
In this example the earpiece speaker 101 comprises a driver unit with no separate enclosure, and the driver unit uses the empty space inside the mobile device 100 as its back volume (i.e. rather than having a separate enclosure forming a back volume). The problem with this kind of design is without a dedicated enclosed space acting as the back volume, the air dynamics may change easily under various circumstances, and it may affect the audio performance. For example, the complexity of the physical layout inside the device, the material used for the case of the device, or the force applied from user movement, for example, tapping to send a message or squeezing to play a game, can all contribute to the audio performance by affecting the air pressure in the back volume formed by the empty space inside the device.
The loudspeaker 102, which is usually placed on the bottom of a mobile device, is generally the main speaker in the acoustic system. The loudspeaker 102 does not usually introduce the same distorted sound problem as the earpiece speaker 101 as it usually has a dedicated sealed enclosure acting as the back volume, and therefore experiences less effects from the bent force or change of the air dynamics. However, in some circumstances, the loudspeaker may also experience similar problems.
FIG. 1b illustrates the earpiece speaker 101 of the device 100 in more detail. In particular, the device 100 comprises a casing 12 and a plurality of perforations 16 (or equivalent thereof) in the casing 12, for providing fluid communication between the inside of the device 100 and a surrounding environment. The earpiece speaker assembly includes a speaker or transducer unit 21 and mounting support 22. The speaker or transducer unit 21 may be attached to the mounting support 22 with a flexible support 23. The mounting support 22 may be attachable to the casing using a mounting adhesive 24 or equivalent means of attachment (e.g. welding, glue bonding, screws, rivets, mechanical interconnections, etc.). The casing 12 defines an enclosure 18 into which the components for use in the device (e.g. electrical components, mechanical components, assemblies, integrated earpiece speaker assembly, etc.) may be placed. The integrated earpiece speaker assembly may be placed adjacent to the perforations 16 such that the speaker or transducer unit 21 separates the perforations 16 from the rest of the enclosure 18 of the device 100 (e.g. effectively forming an air-tight seal between the perforations 16 and the rest of the enclosure 18).
The integrated earpiece speaker assembly may be provided without a well-defined back volume. The back volume for the speaker or transducer unit 21 may be at least partially shared with the rest of the enclosure 18 of the device 100. Thus, the back volume for the speaker or transducer unit 21 may not be fully defined until the integrated earpiece speaker assembly is fully integrated into the final device 100 (e.g. along with all the other components that makeup the CED 10).
An example of an undesirable audio effect, is a buzzing or distorted audio that occurs when a user squeezes the device. The buzzing or distortion is due to the fact that when users apply force and bend the case of the device, air pressure inside the case changes and results in the corresponding change in the back electric motive force (EMF) on the transducer. For speakers, the back EMF is the force which opposes the change in current that induced it. This change of back-EMF results in a direct current (DC) offset of the speaker, and elevates the speaker or transducer within the device, which may lead to collisions between moving speaker membrane and the case of the device. The DC offset causes the excursion of the diaphragm of the transducer or speaker to manifest an asymmetric behaviour, for example the diaphragm moves more in one direction than in the other.
This DC offset may be recovered naturally with the auto-balance of the air dynamics, and the transducer will move back to an equilibrium position. As this natural recovery occurs, the distorted audio effect will slowly decrease until it disappears. For this reason, the worst distortion usually happens at the time of the tapping, squeezing, or releasing event. The intensity of the audio distortion is related to the audio signal being played, and the volume of the audio signal, as well as the pressure level and the rate of change of the air pressure in the back volume. As louder music is played, and as quicker and harder the pressure is applied, the more severe the audio distortion may be.
The audio distortion may be more severe in tablet devices, as tablet devices may suffer from more widely spread pressure, and the air dynamics within the device may therefore recover more slowly than in a smaller/integrated device like a mobile phone. A fast-acting algorithm to detect and cancel the undesired audio effect may therefore be required.
One reactive effect of the change in back-EMF is an increase in the speaker or transducer impedance. The impedance of the speaker or transducer reaches its peak at resonant frequency f0 where the excursion reaches its peak for the same input voltage. One method for detecting a change of the back-EMF of a speaker is to track the resonant frequency f0 of the speaker. However, this method is not reliable and robust enough for the distortion detection for several reasons. First, besides the change of back-EMF, many other factors may contribute to the shifting of the resonant frequency f0, for example, a temperature change, and therefore the change of resonant frequency f0 is not a sufficient evidence for just being caused by the change of the back-EMF. Second, the detection of the resonant frequency requires a complicated algorithm and usually does not meet the real-time requirement for detecting the change in back-EMF in time to compensation for the distortion it will cause. At the same time, it is not practical to obtain an accurate resonant frequency f0 by performing an evenly distributed frequency sweep on the embedded real-time digital signal processor (DSP) system. Instead, normally the audio input is used as the source to realize the resonant frequency tracking. In this case, if the input signal does not have energy at the resonant frequency f0, e.g., if the input signal comprises a tone or signal at specific frequency range that does not cover the resonant frequency f0, then the change of resonant frequency f0 may not be detected.
Given the drawbacks of the method described above, the present disclosure seeks to provide a more robust and fast acting method to compensation for audio distortion related to the change of the back-EMF in a transducer.